This invention relates to color printing, and more particularly to a process for eliminating three-color moiré patterns in four-color (CMYK) printing using parallelogram-shaped halftone cells.
With the advent of inexpensive digital color printers, methods and systems of color digital halftoning have become increasingly important. It is well understood that most digital color printers operate in a binary mode, i.e., for each color separation, a corresponding color spot is either printed or not printed at a specified location or pixel. Digital halftoning controls the printing of color spots, where spatially averaging the printed color spots of all the color separations provides the illusion of the required continuous color tones.
The most common halftone technique is screening, which compares the required continuous color tone level of each pixel for each color separation with one of several predetermined threshold levels. The predetermined threshold levels are stored in a halftone screen. If the required color tone level is darker than the threshold halftone level, a color spot is printed at the specified pixel. Otherwise the color spot is not printed. It is understood in the art that the distribution of printed pixels depends on the design of the halftone screen. For cluster halftone screens, all printed pixels are grouped into one or more clusters. If a cluster-halftone screen only generates a single cluster, it is referred to as a single-cell halftone screen or a single-cell halftone dot.
Alternatively, halftone screens may be dual-dot, tri-dot, quad-dot, or the like.
Halftone screens are typically two-dimensional threshold arrays and are relatively small in comparison to the overall image or document to be printed. Therefore, the screening process uses an identical halftone screen repeated for each color separation in a manner similar to tiling. The output of the screening process, using a single-cell halftone dot, includes a binary pattern of multiple small “dots”, which are regularly spaced and is determined by the size and the shape of the halftone screen. In other words, the screening output, as a two-dimensionally repeated pattern, possesses two fundamental spatial frequencies, which are completely defined by the geometry of the halftone screen. While halftoning is often described in terms of the halftone dots, it should be appreciated that halftone dots can also posses shapes ranging from rectangles, squares, lines, and the like.
A common problem that arises in digital color halftoning is moiré patterns. Moiré patterns are undesirable interference patterns that occur when two or more color halftone separations are printed over each other. Since color mixing during the printing process is a non-linear process, frequency components other than the fundamental frequencies of the two or more color halftone separations can occur in the final printout. For example, if an identical halftone screen is used for two color separations, theoretically, there should be no moiré patterns. However, any slight misalignment between the two color halftone separations occurring from an angular difference and/or a scalar difference will result in two slightly different fundamental frequencies, which will be visibly evident as a very pronounced moiré interference pattern in the output. To avoid, for example, two-color moiré patterns due to misalignment, or for other reasons, different halftone screens are commonly used for different color separations, where the fundamental frequencies of the different halftone screens are separated by relatively large angles. Therefore, the frequency difference between any two fundamental frequencies of the different screens will be large enough so that no visibly noticeable moiré patterns are produced. For three-color separations, it is desirable to avoid any two-color moiré as well as any three-color moiré. Three-color moiré, also known as second-order moiré, occurs when thre halftone screens combine together to produce visible, low frequency colored beat patterns.
It is well known that in the traditional printing industry that three halftone screens, which are square in shape and identical, can be placed at 15°, 45° and 75°, respectively, from a point of origin, to provide the classical three-color moiré-free solution. However, for digital halftoning, the freedom to rotate a halftone screen is limited by the raster structure, which defines the position of each pixel.
Obviously, it is desirable to avoid any two-color and three-color moirés in four-color printing. Usually, in selecting different halftone screens for four color separations, there is only one combination of three screens that will exactly align so that no three-color moiré will be produced, that is there is only one combination of three colors among all four that will meet the three-color zero moiré condition. For example, for a CMYK printer, only the cyan-magenta-black combination will be targeted for three-color moiré free condition. The choice of halftone screen for the four color, yellow, has typically relied on satisfying two-color moiré conditions only and do not provide a moiré free solution for all three-color combinations with yellow.